In this third of a series of studies on experimental dirofilariasis in primate hosts, 3 Macaca cynomolgus, 6 M. speciosa, and 2 M. mulatta were inoculated with infective larvae of Dirofilaria tenuis, a parasite of the raccoon, and were killed at periods ranging from 2 weeks to 13 months of infection. One M. mulatta and one M. speciosa were treated daily with prednisolone beginning 2 months after inoculation; worms were recovered from these and 5 other monkeys (7 of 11). Microfilaremia was detected during the 28th week of infection in a monkey under prednisolone treatment. Microfilaremia was found to be aperiodic in that monkey and in a raccoon. Eosinophilia and antibodies to the microfilaria reached appreciable levels in three monkeys harboring gravid worms. Development of D. tenuis in M. speciosa appeared to be comparable to that observed in man.